Liquid phase fluorination of aliphatic chlorinated derivatives, that is to say the chlorine-fluorine exchange with anhydrous hydrofluoric acid in liquid phase is a known reaction. The most important chlorofluorocarbons, that is to say CFCl.sub.3, CF.sub.2 Cl.sub.2, CHF.sub.2 Cl, and C.sub.2 Cl.sub.3 F.sub.3, can thus be obtained according to a process of this kind by chlorine-fluorine exchange, starting with CCl.sub.4, CHCl.sub.3, and C.sub.2 Cl.sub.6 respectively (J. M. Hamilton, "The Organic Fluorochemicals Industry" in the work Advances in Fluorine Chemistry, vol. 3, 1963, pp. 146-150).
The aliphatic fluoro or chlorofluoro derivatives containing at least two carbon atoms and containing at least one hydrogen atom can be obtained according to the same fluorination process from the corresponding chloro derivatives, but they can also generally be obtained from chloroolefins by reaction with hydrofluoric acid according to a reaction whose first stage is an addition of HF to the double bond. Thus, for example, CF.sub.3 CH.sub.3 can be obtained either by fluorination of CCl.sub.3 CH.sub.3 or by fluorination of CCl.sub.2 .dbd.CH.sub.2 (E. T. McBee, et al., I.E.C., 1947, pp. 409-412). Similarly, CF.sub.3 CH.sub.2 Cl can be obtained by fluorination of CCl.sub.3 CH.sub.2 Cl or of CCl.sub.2 .dbd.CHCl (A. K. Barbour, et al., "The Preparation of Organic Fluorine Compounds by Halogen Exchange" in the work Advances in Fluorine Chemistry, vol. 3, 1963, pp. 197-198).
In some cases, with highly reactive chloro derivatives, it is possible to effect these fluorinations merely by heating the chloro derivative with hydrofluoric acid in the absence of catalyst. Thus, it is known that methylchloroform CCl.sub.3 CH.sub.3 can be converted into CF.sub.3 CH.sub.3 by reaction with anhydrous hydrofluoric acid in liquid phase (E. T. McBee, et al., op.cit.). This reactivity of methylchloroform is, however, quite exceptional and, generally speaking, the reaction of HF in the absence of catalyst does not allow the chlorine-fluorine exchange to be effected or permits only a single chlorine atom to be replaced, and even this at very high temperatures. In practice, liquid phase fluorinations are carried out in the presence of a fluorination catalyst. Various catalysts have been proposed, but the most effective ones have been found among pentavalent antimony halides or mixtures of pentavalent and trivalent antimony halides (Houben-Weyl, Vol. V/3, 1962, p. 126). On an industrial scale, antimony pentachloride SbCl.sub.5 or a mixture of antimony trichloride SbCl.sub.3 and chlorine are generally employed, and these, on reacting with hydrofluoric acid, yield mixed chlorofluorides such as SbF.sub.3 Cl.sub. 2 or SbF.sub.2 Cl.sub.3, which have been found to be particularly effective fluorination catalysts. However, pentavalent antimony chlorofluorides decompose at the temperatures needed for the fluorination and yield trivalent antimony halides and chlorine. Because trivalent antimony halides are practically ineffective in fluorination, catalysts based on antimony 5+ quickly lose their effectiveness and their activity can be maintained only if the antimony is successfully maintained in its 5+ oxidation state. This can be done by reoxidation using chlorine, that is to say by performing the fluorination in the presence of a little chlorine, which enables Sb.sup.3 + to be continually reoxidized to Sb.sup.5+ (Houben-Weyl op.cit.). Nevertheless, catalysts containing antimony exhibit a number of disadvantages:
Mixtures of pentavalent antimony halides and of hydrofluoric acid are highly corrosive, especially at high temperature.
Fluorination in the presence of antimony 5+ is in certain cases accompanied by inconvenient side reactions. Thus, in the case of chloro derivatives containing a hydrogen atom, an olefin can be formed by the loss of HCl, and these olefins can give rise to the formation of heavy products (Houben-Weyl, op.cit., pp. 134-135).
The need for the fluorination to be carried out in the presence of chlorine can also give rise to the formation of a number of secondary reactions. This is the case in particular with the fluorination of chlorinated hydrocarbons still containing one or more hydrogen atoms, which can be replaced by chlorine atoms during this fluorination. In the case where the starting material to be fluorinated is an ethylenic derivative there may be competition between an addition of HF or a chlorine addition to the double bond. In the case of trichloroethylene, it is thus possible to obtain CF.sub.3 CH.sub.2 Cl by HF addition (Houben-Weyl, p. 107) or CF.sub.2 Cl-CFCl.sub.2 by chlorination (Houben-Weyl, p. 134). ##STR1##
Other catalysts for liquid phase fluorination have been proposed. For example, the compounds SnCl.sub.4, MoCl.sub.5, WF.sub.6, NbCl.sub.5, TaF.sub.5, TiCl.sub.4, BF.sub.3, and CF.sub.3 SO.sub.3 H may be mentioned, but these catalysts are generally much less efficient than antimony 5+. Thus, titanium halides have been proposed for the preparation of chlorofluoro-methane or -ethane compounds by fluorination of the corresponding chlorinated derivatives (U.S. Pat. No. 2,439,299). Titanium tetrachloride can also be employed for the fluorination of chloroolefins such as tri- or tetrachloroethylenes (U.S. Pat. No. 4,374,289 and A. E. Feiring, J. Fluor. Chem. 1979, 13, pp. 7-18), for example: EQU C.sub.2 Cl.sub.4 .fwdarw.CFCl.sub.2 --CHCl.sub.2 +CF.sub.2 Cl--CHCl.sub.2 CCl.sub.2 .dbd.CHCl.sub..fwdarw.CFCl.sub.2 --CHCl.sub.2
However, this titanium halide is not a very powerful catalyst, because it generally makes it possible to obtain only monofluorinated or difluorinated products.